Malignantly transformed cells can be viewed as cells frozen in a specific stage of differentiation. Recent studies, by many laboratories, have implicated qualitative or quantitative changes in the expression of a class of genes, referred to as oncogenes, as playing an important role in establishing and maintaining the transformed state. To understand how a change in expression of these genes is related to transformation a more detailed knowledge of the normal expression and function of these genes must be obtained. For instance, to determine if oncogene expression plays a role in determining the phenotype of a cell, it is important to know whether the expression of a particular oncogene is linked to the stage of differentiation of the cell. We have chosen to examine the relationship between specific oncogene expression and the state of differentiation by studying the expression of oncogenes at different stages of erythroid development in the mouse. To do this, we have screened Friend induced murine erythroleukemia cells for the coincident expression of non-Friend related oncogenes. The oncogenes analyzed, myb, myc, erb A, erb B and ets have all been cited as contributing to the transformed state of erythroid cells in avian systems. Only myc and myb showed significant transcription in this system being seen in both Friend virus alone and Friend complex (Friend helper plus spleen focus-forming virus, SFFV) induced erythroleukemias.